The invention relates to a linear light source, and more particularly to such a linear light source for use in a film scanner.
Light integrating chambers for film scanning are known in the prior art. A conventional cylindrical integrating cavity used in a film scanner is disclosed in U.S. Pat. No. 4,868,383 (A. Kurtz et al). This integrating cavity comprised a solid block within which a cylindrical chamber was formed that had diffusely reflecting walls. An intense beam of light was introduced through an input port, and an elongated output slit was provided parallel to the axis of the cylindrical chamber to produce a linear light source. Film was scanned perpendicular to the long axis of the output slit, and the film was imaged onto one or more linear CCD devices.
An important requirement on such illuminators is that they provide diffuse illumination, which is required in order to suppress artifacts that would otherwise arise from scratches and refractive debris (dust) on the film. In practice, the requirement is commonly stated that the radiant intensity of the illumination follow a Lambertian profile out to approximately 45 degrees from the optical axis of the imaging lens (normal to the slit). In ""383, this was best achieved by cutting the slit substantially tangent to the cylindrical cavity, thereby creating a thin, sharp wall near the opening of the cavity.
A preferred material for the construction of integrating cavities is a bulk diffusing material such as Spectralon, available from LabSphere. The bulk nature of the diffusion dictates that a minimum thickness (approximately 1-mm) be maintained for optimum cavity efficiency. Therefore, the cavity configuration of ""383 is not well-suited to this material. In U.S. Pat. No. 5,103,385 (R. Federico et al), the diffuse illumination is provided with a substantially cylindrical integrating cavity with linear sidewalls adjacent to the slit and disposed at an angle of approximately 45 degrees to the optical axis of the imaging lens.
Another integrating chamber that produces a desired diffuse angular profile is disclosed in U.S. Pat. No. 5,215,370 (M. Kaplan), wherein plates are inserted in the cavity to create a channel to direct light out into a predetermined angular range.
Other approaches for suppressing scratches and refractive defects on the film have been disclosed using processes wherein the defects are detected and removed from the scanned images using image processing techniques. A process for detecting defects using illumination of infrared light is disclosed in U.S. Pat. No. 5,266,805 (A. Edgar). Another process for detecting defects, using dark-field illumination, is disclosed in U.S. Pat. No. 5,969,372 (D. Stavely et al). Unfortunately, the image manipulation required to correct for the detected fringes makes these approaches difficult to implement in high-data-rate applications, such as high-resolution, real-time scanning of motion picture films.
A second important requirement that has not been explicitly treated in the patent art is the suppression of artifacts due to contamination of the illumination system near the film. This can arise due to dust or other debris falling into an integrating chamber, and the defects can be opaque or translucent but refractive.
The Spirit telecine is a motion picture film scanner that is marketed by Thomson using a Kodak optical system. This system comprises a set of cylindrical integrating cavities to illuminate a film sample and a set of lenses to image the film sample onto linear sensors. For a given film format to be scanned, a combination of integrating cavity and imaging lens is selected by the user. The diameter of the cylindrical integrating cavity was selected to give the desired illumination characteristics independent of the imaging lens, as disclosed in U.S. Pat. No. 5,241,459 (M. Kaplan et al).
A third requirement for an illumination system is that the efficiency be as high as possible while still providing the desired uniformity and diffusion. This can be achieved by designing the cavity to have the highest possible reflectivity of the walls and the minimum possible area for the introduction of the light. Bulk diffusers such as Spectralon currently have the lowest loss of available reflective diffuse materials. Furthermore, an integrating cylinder apparatus that allows a small interior surface area and entrance hole area while maintaining a desired intensity profile by using a light pipe to couple light into the cavity has been disclosed in U.S. Pat. No. 5,274,228 (M. Kaplan).
The aforementioned Spirit telecine directly inputs light through apertures in the sidewalls of the integrating cylinders, that is, without the use of a light pipe. In the Spirit telecine, the cylinder cavities are sufficiently large in diameter such that, together with the particular f/#s of the lenses for the film formats in use, the resulting system has been found to effectively mask artifacts due to contamination of the illumination system. However, the integrating cylinders are rather large in surface area and, as explained above, it is well known that the efficiency of the light source will increase with smaller internal surface areas. Moreover, as the surface areas decrease, it was found that the contamination-masking effect observed in the system was lost.
It is an object of the invention to provide a line illumination for a film scanner that suppresses artifacts due to scratches and dust on the film as well as within the illumination system. It is a further object of the invention to provide a high degree of uniformity while maintaining a high efficiency.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a light integrator for producing diffuse illumination from a beam of light wherein image artifacts due to debris within the integrator are suppressed. The light integrator includes an integrator block having an elongated cylindrical light integrating cavity enclosed by end walls and a longitudinal cylindrical chamber wall having a diffusely reflecting interior surface. The chamber wall includes a longitudinally extending output slit for emitting light from the cavity. A dust-free zone exists within the cavity in the shape of a sector of a circle within which contaminants cannot come to rest without producing visible artifacts, wherein the origin of the sector is located at or nearby a plane of the original. An elongated light pipe extends into the cavity through one of the end walls, where the light pipe has an input port at one end thereof for introducing the beam of light and a treatment along its length for emitting light entering its port into the cavity. At least one of the end walls forms a support for the elongated light pipe so that the light pipe extends along a length of the integrating cavity outside of the dust-free zone and in relation to the chamber wall thereof to direct light emitted therefrom toward the diffusely reflecting interior surface.
In an alternative aspect of the invention, the dust-free zone is formed within the cavity in the shape of a sector of a circle within which particle contaminants cannot come to rest without producing visible artifacts, wherein the origin of the sector is located at or nearby a plane of the original and wherein the radius of the dust-free zone does not reach the chamber wall. Furthermore, the light integrator is used in connection with an imaging lens including an entrance pupil defined on an optical axis that allows light rays directed up to a predetermined angle xcex8l from the optical axis to pass through the illuminated section of the original, thereby defining an f-number equal to twice the tangent of the predetermined angle xcex8l, and the radius of the dust-free zone is a function of a size of the particle contaminants and the f-number.
The advantage of the invention is that it provides a line illumination for a film scanner that suppresses artifacts due to scratches and dust on the film as well as within the illumination system, all within a very small geometry. It is a further advantage that it provides a high degree of uniformity while maintaining a high efficiency.